Grain size and composition effects on the cathodoluminescent characteristics of sprayed zinc oxide thin films

Abstract

The cathodoluminescent (CL) property of 1 µm-thick sprayed ZnO films is here reported versus the sample mean grain size (D) resulting from the spraying flow rate (f) parameter. The deposition process, performed at T _s = 723 K, leads to hexagonal packed structure of [002] preferred growth orientation, regardless of the f value. The CL-spectra are typical of the film's growth rate (r) and the intensity (I) of the three main emission lines (λ ≈ 386, 497 and 644 nm) is greatly affected by D that is ranged between 0.40 and 0.90 µm. Their (I − D) curves have a similar profile that presents a common critical point, D _c = 0.57 µm, depicting two different luminescent behaviours of the material. Smaller grains exhibit optimal intensities. Their band-edge near UV line (λ ≈ 386 nm) proceeds from a phonon replica phenomenon; excess of interstitial Zn explains the result of their blue–green line (λ ≈ 497 nm) while deep-level interactions effect is the relevant interpretation for the red one (λ ≈ 644 nm). A decrease of luminescent properties from D = 0.45 µm till D _c is assigned to the sample microstructure degradation. The bell-shaped profile of CL-intensity for D>D _c reflects a competition between the increase of the material reactive part and the collapse of crystallites quality due to higher content of Cl and S impurities in bigger grains. A limit of the two approaches appears at D = 0.65 µm.